There are bugs that are not being looked at due to lack of people in different areas.

If you have bugs that you would like to see fixed, the best place is bugreports and codereview. Providing help there is appreciated. Ideas on how to fix things help, and even better if you can fix a bug an commit it to the codebase.

Yes that is correct. Every of the 8 custom gestures defines its own set of data in addition to the QGestureState, which is set by the according gesture recognizer.

I now implemented an abstract superclass for all these gestures with a method getGestureData(). This method basically wraps all the data in a QMap<QString,QVariant> to make a general gesture treatment possible. The custom event GestureEvent now takes gesture type, the Qt::GestureState and the mentioned gesture data map.
It was a little bit circumstantial, but now it works.

I found out the solution using native Windows Gestures WM_Gesture.
By default QT registers QMainWindow-Window as a Touch Window, so the QMainWindow-App only get WM_Touch events.
As said above one can only get either WM_Touch event or WM_Gesture event. So you have to unregister the window from getting Touch event. I do that in the constructor like this: